The invention relates to a dynamic mixer for viscous compositions, in particular for components of dental impression compounds.
It is known (EP-B-492412) to attach a dynamic mixer to a device for dispensing the components which are to be mixed, said dynamic mixer having at the other end an outlet opening for the mixture. It comprises a mixer tube, a rotor which is mounted and can be driven rotatably therein and which, with the mixer tube, delimits a mixing channel of annular cross section, and an end wall with inlet openings through which the components to be mixed pass into the mixing channel. The rotor has mixer blades which are intended to mix together the components flowing through the mixing channel.
The object of the invention is to improve the mixing effect. Accordingly, the rotor has one or more chambers which are completely or at any rate for the most part open towards one side. These open sides are directed towards the inlet openings in the end wall, so that the components passing through these openings into the mixer are initially received by the chambers. This can involve a single chamber arranged round the entire circumference of the rotor, or a plurality of chambers. Where one chamber is referred to in the text below, this reference will also always cover embodiments having a plurality of chambers, and vice versa.
As they move past the inlet openings, the chambers alternately receive partial quantities of the components, which stack up on each other or accumulate in the chambers. They pass, pre-mixed and/or in rapid alternation, into the mixing channel through outflow opening(s).
The underside of the chamber is advantageously designed as a plate forming the outflow opening or outflow openings. The plate forms the outflow opening(s) advantageously by way of cutouts through which the components are pressed. These cutouts can be of circular shape, for example, or in the form of incisions designed preferably as radially oriented slots. They are expediently distributed uniformly over the underside of the chamber so that deposits not participating in the flow process are minimized.
However, the plate can also form an outflow opening by means of the outer radius of the plate being slightly smaller than the inner radius of the mixer tube. The outflow opening is in this case formed by a gap between the plate edge and the inner surface of the mixer tube. Between the moving outside of the plate and the stationary mixer tube, the composition pressed through this gap is subjected to a shearing stress which promotes mixing.
A plurality of chambers distributed about the circumference are advantageously separated by walls. Distributing the chambers about the whole circumference has the effect that all of the components undergo the stacking process. The walls in the first place ensure more uniform mixing and in the second place ensure that the components emerging from the inlet openings are entrained in small portions.
The total surface area of the outflow opening(s) in relation to the cross-sectional area of the mixing channel is preferably 10 to 50%. Insofar as the cross-sectional area of the mixing channel varies along the mixer tube, this means the cross-sectional area of the mixing channel at the level of the outflow opening(s). With these size ratios, it has been found that, when the components are pressed out through the outflow opening, very good mixing occurs. The mixing is particularly good in the case of a ratio of 25%.
Mixer blades which further improve the mixing are advantageously arranged on the rotor. The mixer blades can be arranged in groups, in which case the mixer blades of one group are each located at the same height of the axis of rotation of the rotor. As regards the mixing result, it is particularly advantageous if the distances between the groups vary along the axis of rotation of the rotor.
Of the area of the rotor under the chamber(s), a length section of at least 10% and expediently not more than 40%, preferably between 15% and 25%, is advantageously free of mixer blades. It has been found that in this area there is substantial interplay between the composition and the inner wall of the mixer tube. This improves the mixing result compared with a continuous arrangement of mixer blades.
Stationary blades are advantageously arranged on the wall of the mixing channel, at the level of the rotor area which is free of mixer blades. This further enhances the interplay between the inner wall of the mixer tube and the composition.
So-called baffles are advantageously arranged on that side of the end wall directed towards the mixing channel, through the inlet openings of which the components pass into the mixing channel. In this way, the free flow of the components in the area between the end wall and the edges of the walls separating the chambers is impeded, thus preventing the components from hardening already in the area of the inlet openings. The mixing is additionally improved because the baffles modify the flow paths and thus effect different flow rates. Where such baffles are present, the walls of the chambers do not strip the components from the walls of the inlet openings but from the baffles.